This invention relates to magnetic recoding media, and more particularly, to magnetic recording media having a base film plasma treated for improved durability.
Several decades have passed since the appearance of megnetic recording media comprising a nonmagnetic substrate and a magnetic layer formed from a magnetic powder of oxide type such as gamma-Fe.sub.2 O.sub.3 and cobalt-containing gamma-Fe.sub.3 O.sub.4 and a binder. Recently, for the purpose of further improving recording density, magnetic recording media comprising a magnetic layer formed from a ferromagnetic powder such as Fe, Co, Ni, Fe-Co, Co-Ni, Fe-Co-Ni, Fe-Co-B, Fe-Co-Cr-B, Mn-Bi, Mn-Al, Fe-Co-V, etc. and a binder, and magnetic recording media having a magnetic layer in the form of a vacuum deposited metal film or sputtered metal film were developed and are now of great interest in the art.
These magnetic recording media, particularly for use as magnetic tape and magnetic disc, must fulfill a number of requirements including low dynamic coefficient of friction, smooth and stable travel performance for a prolonged period, improved wear resistance, stability under storage environment to ensure consistent reproduction, and durability (durability of tape both during normal operation and in the still mode).
A variety of pre-treatments have heretofore been made on various base films for the purpose of improving durability. Such pre-treatments include treatments with chemical solution, coating, corona discharge treatment, and the like.
Chemical treatments may be treatments with acid and alkali. Among such chemical treatments most effective is by oxidizing the surface of a base film with a chemical solution of a strong acid and/or a strong oxidizing agent, for example, chromate solution, and introducing carbonyl or carboxyl radicals to etch the surface. The chemical treatments, however, require subsequent rinsing and drying of film surface and a great investment is needed for the treatment of spent liquid. Particularly, chromate treatment yields a spent liquid which must be carefully treated for environmental pollution control, and its commercial utilization is now diminising.
The film coating technique is by coating a base film with an undercoat on which a magnetic layer is formed. The interaction between a binder in the undercoat and the magnetic layer is necessary. The composition of the undercoat must be selected to meet a particular binder and pigment used in the magnetic layer. The coating techniques require not only such a careful choice, but also coating and drying steps. Of course, the consumption of coating material increases the cost of products.
The corona discharge treatment is advantageous because of dry nature eliminating the need for additional steps of rinsing, drying, and disposal of spent liquid. Corona treatment has been carried out for many years and is effective in improving adhesion, wettability, and printability. The corona treatment, however, is not successful in improving the properties of magnetic recording media to such an extent as to fulfill the high performance which is imposed on the present day and future magnetic recording media.
Another technique known in the art is a flame treatment which is difficult to apply to magnetic recording media which require a high degree of dimensional stability.
Under these circumstances, a proposal is made to treat base films with a plasma. The plasma treatment is a one-step dry process and thus has the advantage that drying and disposal of spent solution are unnecessary and no extra material like binder is consumed. In addition, the plasma treatment enables high speed, continuous production so that it can be readily incorporated in the process of manufacturing magnetic recording media without sacrificing production speed and yield.
One technique for plasma treatment of base film is disclosed in Japanese Patent Publication No. 57-42889 (published on Sept. 11, 1982) wherein a treatment is effected with a plasma having a frequency in the range of radio frequency to microwave using a treating gas of air, oxygen, nitrogen, hydrogen, helium, argon, etc. The radio frequency of 13.56 MHz is only described in this publication.
Also, Japanese Patent Application Kokai No. 58-77030 (laid open on May 10, 1983) discloses a process of plasma treatment by applying an AC current at the commercial frequency between electrodes using a treating gas of oxygen, argon, helium, neon or nitrogen. These plasma treatments are somewhat successful in improving the adhesion of a treated base film to a magnetic layer and hence, the durability of magnetic recording media, but not fully satisfactory in bond strength and durability.
When it is intended to provide a magnetic thin film layer for perpendicular magnetization recording by applying a magnetic metal such as iron, cobalt, nickel and chromium or an alloy of such metals onto a substrate by vaccum deposition, sputtering or similar techniques, the substrate must be kept at an elevated temperature. Since substrates of conventional polyester resins, however, have a relatively low heat-resistant temperature and are liable to heat distortion during film formation, they should not be heated at an elevated temperature.
Thus, heat distortion during film formation may be avoided by the use of heat-resistant resins such as polyamides and polyimides as substrates for supporting perpendicular magnetization films. Unfortunately, most heat-resistant resins contain water therein. During film formation at elevated temperatures, the water content in the resin can be released to disturb the crystal structure of the magnetic layer being formed, detracting from perpendicular magnetization properties.